Direct bridge rectification of the ac input line into a capacitive filter results in a poor power factor and high harmonic current pollution. This is no longer acceptable due to tightened international regulations governing the power quality and harmonic current pollution of the utility input line from the ac source. Thus, same form of active power factor control has become necessary.
Recent international regulations governing the power quality and harmonic current pollution of the utility by users has placed increased emphasis on the problem of the interfacing electronic loads to the utility line via power conversion circuits. By using an ac rectifier and a dc-to-dc converter for active input current shaping, it is possible to achieve unity power factor (UPF) operation out of such a dc-to-dc converter while providing regulated voltage to a load. Low frequency energy required for input-output power balance is usually stored externally in an output capacitor connected across the load.
The main drawback of an input current shaper comprising the dc-to-dc converter with external energy storage is low bandwidth of the output voltage regulation limited to a few hertz. This is a consequence of the need to have a large external output capacitance and having a single control in the dc-to-dc converter which cannot simultaneously provide both input current shaping and fast output voltage regulation.
One known solution for achieving UPF operation and at the same time wide bandwidth regulation of the output voltage to a load RL is shown in FIG. 1. The technique is to use a dc-to-dc converter 10, such as a forward, flyback, Cuk, Sepic, half-bridge or full-bridge converter, for output voltage regulation with an additional front-end switching regulator 11 for input current shaping, usually a boost converter coupled to an ac source 12, typically a public utility line, by a fullwave rectifier and lowpass filter 13. Low frequency energy is stored in a capacitor C1 between the two regulators. Galvanic isolation and fast output regulation are then provided by the down-stream dc-to-dc switching converter 10. This solution has several serious drawbacks: (a) power is processed twice, thus leading to lower overall conversion efficiency, (b) the complexity of such a UPF switching converter is increased and (c) both size and cost are increased.